EASY ANSWERING BIOLOGY : MOVEMENT OF SUBSTANCES ACROSS THE PLASMA MEMBRANE

Describe the structure of the plasma membrane based on model proposed by S.J. Singer and G. Nicholas.

• Plasma protein is composed of phospholipid and protein.
• Phospholipid molecule has polar head whish is hydrophilic
• and non-polar tail which is hydrophobic.
• Phospholipid bilayers are not rigid or static but form a dynamic flexible structure (that is always changing).
• Various types of protein are partially or wholly embedded in the membrane.
• There are pore protein and carrier protein.
• Protein molecules float about in the phospholipid bilayer to form a mosaic pattern that is always changing.
• Contains cholesterol which links the fatty acids together.
• Cholesterol helps to stabilize/ strengthen plasma membrane.

State characteristics of the phospholipid bilayer.

• It consists of hydrophilid heads and hydrophobic tails.
• The hydrophilic heads are facing the exterior and the hydrophobic tails are facing the interior of the cell.
• The bilayer is dinamic / not static.

Explain how certain substances pass through the plasma membrane.

• The phopholipid bilayer is made up of a polar head and non-polar tail.
• The head of phospholipid is hydrophilic while the tail is hydrophobic.
• Polar molecules like water can cross the phospholipid layer by osmosis.
• Small neutral molecules like oxygen / carbon dioxide can cross the lipid layer by simple diffusion.
• Small lipid-soluble molecules can also cross the lipid layer.
• Large molecules like glucose and amino acids across the plasma membrane with the help of carrier proteins.
• Small molecules with charges slightly larger molecules across the membrane using pore protein.

Explain the similarities and differences between facilitated diffusion and active transport.

SIMILARITIES

• Both ways of transportation need carrier protein
• to bind with glucose / sodium ion

• Both transport specific molecules only
• because carrier protein have specific sites ( to certain molecules)

• Both processes occur in living cell
• because they need carrier protein which can change shape to allow substances to move across.

• Both processes transport substances which cannot pass through the phospholipid layer of cell membrane.

DIFFERENCES

Aspect	Facilitated diffusion	Active transport
Movement of molecules	Goes down the concentration gradient. From a region of higher concentration to a region of lower concentration	Goes against the concentration gradient. From a region of lower concentration to a region of higher concentration
Molecules can move through	Pore proteins & carrier proteins	Carrier proteins only
Energy (ATP)	No energy is used. Cellular energy is not required to transport glucose.	Energy is used. Cellular energy is required to transport ions.

Describe how ions are transported in the cell.

• By active transport- The movement of ions is against the concentration gradient.
• And it needs energy which is produced by cellular respiration (in the form of ATP molecules).
• Carrier proteins bind with the ions and change their shape, thus carrying the ions across plasma membrane.

A chemical substance inhibits the respiration process in the root hair cells of plant. Explain the effect to the transport of mineral ions into the root hair cells.

• The cell unable to produce energy.
• Active transport does not occurs.
• Mineral ions cannot be transported into the cell.

Explain the movement of water molecules across plasma membrane by osmosis.

ISOTONIC SOLUTION (5 % sucrose solution)

• The cell retains its normal shape.
• The 5% sucrose solution concentration is isotonic to the sap cell of vacuole.
• Water molecules diffuses into and out the cell at equal rates.
• There is no net movement of water across the plasma membrane.
• The protoplasm does not change/ Vacuole does not expand or shrink.

HYPOTONIC SOLUTION (0.1% sucrose solution)

• The cells become turgid.
• The 0.1% sucrose solution concentration is hypotonic to the sap cell of vacuole.
• Water molecules diffuse into the sap cell of vacuole by osmosis.
• The vacuole expand & swell up / The plasma membrane pushes against the rigid cell wall / Create turgor pressure

HYPERTONIC SOLUTION (30% sucrose solution)

• The cells become shriveled / undergo plasmolysis.
• The 30% sucrose solution concentration is hypertonic to the sap cell of vacuole.
• Water molecules diffuses out from the sap cell of vacuole
• by osmosis.
• Both the vacuole & cytoplasm shrink / The plasma membrane pull away from the cell wall.

Explain the method used to preserve fish.

• Salting - The fish is immersed in the salt solution which is hypertonic to the fish tissues.
• More water diffuses out from the fish tissues into the salt solution by osmosis.
• The hypertonic solution causes the fish to be dehydrated.
• This prevents bacterial growth in the fish tissues.
• The bacterial cells are also plasmolysed.
• This prevents decay of the fish.

Describe the method in preserving vegetables.

• Vegetables are immersed in vinegar which is acidic.
• Vinegar diffuses into the vegetables tissues.
• The vegetables tissues become acidic.
• The low pH prevents bacterial growth on the tissues.
• This prevents decay of the vegetables.

Explain the condition of plant after added with excess fertilizer.

• Excess fertilizer will cause the soil water to be hypertonic towards the root hair cells.
• Water from the root hair cells diffuse out to the soil by osmosis.
• The cells become plasmolysed and this leads to wilting.